One to Many Relationship – FastAPI - Beginner to Pro

🔗 What is a One-to-Many Relationship?

When discussing SQL tables, a one-to-many relationship means:

One record in a table can be related to multiple records in another table,
but each of those records belongs to only one parent record.

In our application:

A single user can have multiple todos.
A single todo cannot belong to multiple users.

If you are a user, you might have:

“Take out the garbage”
“Get a haircut”
“Take a shower”

All of these todos belong to you — and only you.

At the same time, the system can contain many users, each with their own list of todos.

That is the meaning of:

✔ One User

✔ Many Todos

🗄️ Users & Todos — Two Separate Tables

From a backend and database design perspective, we are working with two separate tables.

1️⃣ Users Table

This table stores authentication and identity data.

id (PK)	email	username	first_name	last_name	hashed_password	is_active
1	codeveda@gmail.com	codeveda1	code	veda	13ieidq80hscd…	1
2	example@gmail.com	example	example	user	uqwb38223jd…	1

Important Concept

PK (Primary Key) uniquely identifies each user.
No two users can share the same primary key.
The primary key is how the database distinguishes records internally.

2️⃣ Todos Table (Before Relationship)

Originally, our todos table looked like this:

id (PK)	title	description	priority	complete

But here’s the problem:

👉 There is no connection between a todo and a user.

So how does the database know which todo belongs to which user?

It doesn’t.

🔑 Introducing the Foreign Key

To connect the two tables, we must add a new column to the Todos table:

`owner`

This column will store the user’s primary key.

Now our todos table becomes:

id (PK)	title	description	priority	complete	owner (FK)

What Does FK Mean?

FK = Foreign Key
A foreign key references a primary key in another table.
It creates the relationship between tables.

In our case:

todos.owner  →  users.id

This means:

The owner column in the todos table references the id column in the users table.

📊 Real Example With Data

Let’s look at actual data.

Users Table

id (PK)	email	username
1	codeveda@gmail.com	codeveda1
2	example@gmail.com	example

Todos Table (With Foreign Key)

id	title	priority	owner (FK)
1	Go to store	4	1
2	Haircut	3	1
3	Feed dog	5	2
4	Water Plant	4	2
5	Learn something New	5	1
6	Shower	5	2

🔎 Understanding What’s Happening

Let’s analyze this carefully.

All records with:

owner = 1

Belong to:

users.id = 1

That is → codeveda1

So todos 1, 2, and 5 belong to user 1.

All records with:

owner = 2

Belong to:

users.id = 2

That is → example user

So todos 3, 4, and 6 belong to user 2.